DosDupHandle enables a process to create a duplicate handle for an open file, pipe, or device.

The value for the old-file-handle parameter is the handle of an open file, a pipe, or a device. The valid values for the new-file-handle parameter include FFFFH, 0000H (standard input), 0001H (standard output), and 0002H (standard error). Any value other than FFFFH is assumed to be the value of the new file handle.

A value of FFFFH causes the system to allocate a new file handle and send it to this location. If the value specified for the new-file-handle parameter is that of a currently open file, the file handle is closed before it is redefined.

An agreed upon value for a duplicate file handle can be established between a parent process and a child process. Avoid choosing arbitrary values for the new file handle.

The duplicate handle acquires the characteristics of the original. If you move the read/write pointer of the original file handle, for example by calling DosRead, DosWrite, or DosSetFilePtr , the pointer of the duplicate handle is also moved. If the original handle has access to regions in a file that have been locked by DosSetFileLocks , the duplicate also has access.

If inheritance was indicated when a file was opened with DosOpen, a parent process can create a duplicate handle for the file and pass it to a child process by means of shared memory. This permits the child to close the duplicate handle without affecting the file handle of the parent.

Because a parent process controls the meanings for standard I/O done by any child process it creates, the parent can use DosDupHandle to redefine unnamed pipe handles as standard I/O handles to communicate with a child. The steps involved are:

• The parent process creates two pipes and duplicates the read handle of one pipe as 0000 and the write handle of the other pipe as 0001.

• When the child process performs standard I/O, instead of reading from the keyboard and writing to the display, it reads from and writes to the pipes created by its parent.

• As the owner of the pipe, the parent process uses its read and write handles to write to the pipe defined to the child as standard input and read from the pipe defined to the child as standard output.